How Planets Work

Congratulations!

You are now the proud owner of an Acme planet, the finest planets in the
galaxy. Whether it's a rocky terrestrial planet, a gas giant, or an ice planet,
your Acme planet is guaranteed to give you billions of years of satisfaction.

Your planet comes with a durable factory finish. You can keep the finish
pristine for eons by following a few simple rules. Protect the surface from
harsh abrasives like large impacting asteroids. Avoid sources of excessive heat,
like exploding supernovae. Melting or vaporizing your planet voids the warranty.

If your planet is stored in a moderate temperature setting, it may become
coated with a thin film of liquid water. This is normal and does not constitute
a malfunction. Cooler parts of your planet may become coated with frozen water,
this too is normal and does not constitute a malfunction. Under some
circumstances, your planet may become covered with a thin greenish film called
life. Connoisseurs consider this the best part.

Bulk Density

Even if we don't have any samples of a planet, we can tell a lot about it
from its bulk density. Bulk density is the mass of the planet divided by
its volume. We know the mass by observing the planet's effects on other bodies
or - best of all - spacecraft tracking. We determine the volume from the size
and shape of the body. Earth-based measurements will do if we don't have
spacecraft data, but spacecraft data are far better.

Density less than 1 gm/cc (1000 kg/m3): If it's a small body
like a comet nucleus, it's probably ice with a lot of void space, like
crunchy snow. If it's a giant planet, like Saturn, it's mostly gases. Saturn
has a bulk density of only 0.7 gm/cc (700 kg/m3) - it's light
enough to float.

Density 1-3 gm/cc (1000 - 3000 kg/m3): Small bodies are made
of mixtures of ice and rock, with higher densities pointing to more rock.
Large planets like Jupiter, Uranus and Neptune are mostly gases with dense
cores.

Density greater than 5 gm/cc (5000 kg/m3): Rock, with a large
dense core of metal.

Shape

Why Are Planets Round?

Planets are round because of gravity. Gravity pulls everything inward toward
the center of a planet.

The pressure inside a planet is simply due to the weight of the overlying
material, pulled inward by the planet's gravity. The stronger the gravity, the
greater the pressure. A meter thick layer of rock exerts more pressure on earth
than it would on the Moon, because of the earth's greater gravity. If the
pressure deep in a planet is greater than the strength of the rocks, the rocks
can flow. High temperatures make the rocks flow more easily. If we have a large,
irregular body, the pressures deep in the interior will cause rocks to flow out
from under high spots (where the pressure is greater) and into low spots (where
the pressure is less). High bulges will sag inward, and the floors of
depressions will flow outward.

In the animation above, let's pretend we're trying to build a square planet.
If the planet ends up as big as the earth (blue circle), the original square
will be 11,300 km on a side. The corners of the square are mountains over a
thousand kilometers high and the sides in between are hundreds of kilometers
below the eventual surface of the planet.

The pressures under the corners are so enormous that they sink into the
planet (yellow arrows) and squeeze material outward in between. The process
finally stops when the differences in pressure inside the planet are small enough that the
rocks of the planet can withstand them.

Equatorial Bulges

Fast rotating planets are not spherical, but bulge at their equators because
of "centrifugal force." The earth's equatorial bulge is too small to be seen in
spacecraft images, and indeed many commercial globes are more out of shape than
the real earth. The earth's equatorial bulge is 1/298 of its diameter, so the
radius at the equator is about 11 kilometers more than at the poles. One
implication of this difference is that the Ecuadorian volcano Chimborazo and the
Peruvian peak Huascaran are the
farthest points from the center of the Earth, not Mount Everest.

Faster-rotating planets have larger equatorial bulges. Jupiter and Saturn are
about 10% greater in equatorial diameter than polar diameter.

Gravity

Studies of a planet's gravity can furnish important clues about the interior
of the planet. The path of a satellite around a planet is slightly affected by
masses within the planet. Satellite tracking has been used to study crustal
thickness and deep rock masses for the Moon, Mars, and Venus and even Saturn's
moon Titan, and of course this information is of critical importance for
accurate satellite navigation on Earth (also missile targeting).

There's another clue we can derive from spacecraft tracking, called Moment
of Inertia. If you've ever turned a playground carousel with children on it,
you know it's easier to start and stop if the children are bunched in toward the
center. The quantity that describes how rotating objects behave is called moment
of inertia. If we can determine the moment of inertia of a planet, we can tell a
lot about how mass is distributed within the planet.

But how do we determine the moment of inertia? If the body precesses, like
the earth, we can determine the moment of inertia from the precession.
Otherwise, moment of inertia affects the shape and size of a planet's equatorial
bulge, and it can also be determined if we have enough measurements of the
planet's gravity from spacecraft observations.

Atmospheres

High in the fringes of a planet's atmosphere, individual molecules obey the
same laws of physics as spacecraft. If they're moving faster than the escape
velocity of the planet, they can escape.

So two things dictate whether atoms can escape from a planet:

Escape velocity, which is determined by the mass of the planet.

The speed of the atoms, determined by:

Temperature

Other sources of acceleration, like fast-moving particles from the
Sun.

The more massive a planet is, the more likely it is to retain an atmosphere.
However, Mercury, with a mass 0.055 that of earth and an escape velocity of 4.25
km/sec, lacks an atmosphere, while Saturn's huge moon Titan has a mass 0.0225
that of earth, an escape velocity of 2.64 km/sec, and a very significant
atmosphere, actually denser than that of earth. Obviously mass alone can't be
the sole determining factor.

Temperature is also important. The average velocity of molecules in a gas at
is 145.5 ˆš(T/m) where T is temperature (in
formulas, T is almost always Kelvin) and m is the molecular mass. For nitrogen
at room temperature, m = 29 and T = 298 K, and the velocity is about 466 meters
per second. On Mercury, where maximum temperature is about 700 K, the average
velocity of a hydrogen molecule (m=2) is 2700 meters per second or 2.7
kilometers per second. Now that's less than Mercury's escape velocity, but still
about half. And a significant number of molecules are moving much faster than
average - about 2% are moving twice as fast and one in 10,000 moving 3 times as
fast. A pretty substantial number will be moving fast enough to escape. Once
they're gone, they're gone, and the remaining molecules, also heated by the sun
up to 700 K, will still have some that exceed escape velocity, and so on.
Mercury can't hold on to hydrogen very long. For carbon dioxide (m = 44), the
average velocity is only 580 meters per second.

For Titan, T = 94 K. For hydrogen, average velocity is about 1000 meters per
second, close enough to Titan's escape velocity not to hang around all that
long. For nitrogen (m = 29), the dominant component of Titan's atmosphere, the
average velocity is a sluggish 261 meters per second, and for carbon dioxide (m
= 44) it's only 212. So Titan can hold on to these heavier gases.

But there's more to it than that. The earth's outermost atmosphere has
temperatures above 1000 K even though the average surface temperature is about
290 K. High energy light from the Sun and charged particles (the "solar wind")
accelerate these atoms to high speeds. Fortunately, since Earth's escape
velocity is a hefty 11 km/sec, it's still not enough. But on Mercury the sun
would accelerate molecules near the top of any atmosphere to very high speeds,
and a lot would escape. So even if Mercury had a carbon dioxide atmosphere to
begin with, and a high enough escape velocity to hold it, the Sun would still
strip it away eventually. On Titan, much farther from the Sun, the Sun would be
far less effective at stripping off an atmosphere.

(Since hydrogen is so light and escapes so easily from planets, do we have
anything to fear from replacing fossil fuels with a hydrogen economy? Not much.
First, the hydrogen will be kept in containers and very quickly burned to create
water vapor, returning to its original state. Any that does escape will move at
the low velocities typical of the lower atmosphere, and will quickly combine
with oxygen in the air (see Hindenburg, 1937). So effectively none of it
will get high enough to be accelerated to escape velocity.)

Early in their life cycles, stars go through a phase called the T Tauri
phase (after a star that is in that phase now) where it blasts off tremendous
amounts of particles at very high speeds. If the inner planets early in the
history of the solar system had dense atmospheres, most or all of it would have
been torn off by the powerful T Tauri solar wind. But planets in the outer solar
system would have been much less affected.

Oceans

On earth, oceans are made of water, but there's no natural law that says an
ocean must be made of water. On Saturn's moon Titan there is good
evidence for large lakes of the hydrocarbon ethane. We could imagine a very cold
planet with seas of liquid nitrogen. But regardless of the liquid, there are
physical laws that govern whether a planet can have an ocean.

People who have camped at high altitudes know that water boils at lower
temperatures. At altitudes around 20 kilometers, it boils at body temperature,
so pilots of high altitude aircraft must wear pressure suits. And in a vacuum,
water would boil away entirely.

An ocean on a planet without an atmosphere would evaporate. The vapor would
remain around the planet as a temporary atmosphere, but pretty soon whatever
caused the planet to lack an atmosphere - low mass, high temperature, or
stripping by solar wind - would cause the vapor to leak into space, and the
ocean would be gone completely. So the first rule for an ocean is that the
planet must have an atmosphere, meaning the planet must be massive and cool
enough to retain an atmosphere.

Also, liquids are an "in between" state between solids and gases, and only
exist in a certain temperature range. Solids can never be too cold to be solid
and gases can never be too hot to be gases, but liquids can both freeze and
evaporate. So a planet has to have a temperature that
significantly overlaps the liquid range. Temperatures can occasionally fall
below the freezing point (as on earth) or perhaps exceed the boiling point, but
the planet must remain mostly within the liquid range. So we can expect seas,
lakes, and oceans on planetary surfaces to be fairly uncommon in the universe.

But another possible way for a planet to have an ocean is to have an
internal ocean. Most small bodies in the outer solar system have rocky cores
and icy surfaces. The rocky cores have small amounts of the radioactive elements
uranium and thorium, and ice is a pretty good insulator. It doesn't take much
heat production from a small rocky core to melt ice below the surface. It is
widely believed that Jupiter's satellite Europa has a subsurface ocean, possibly
shallow enough to probe. Subsurface oceans could be common on icy worlds. Lack
of an atmosphere won't affect an ocean that's sealed beneath a solid shell,
though there might be a tiny loss if liquid ever leaks to the surface
occasionally.

Even if a planet has an ocean, it may not be able to retain it. Stars
brighten with age, and a planet that may have an ocean early in its star's
lifetime could lose it later on. If evaporated molecules from the ocean make it
high into the atmosphere, they may be broken apart by a process called
photodissociation, where ultraviolet light knocks atoms off the molecule.
For example, water molecules can be split into hydrogen and oxygen, and since
hydrogen is so light, it can easily escape. If the planet gets so hot that
evaporation is very extensive, a planet can lose an ocean pretty quickly. Many
astronomers think Venus may once have had an ocean but lost it this way. Present
theories of solar evolution predict the Earth will become too hot to hold oceans
in one or two billion years. Freezing, on the other hand, is no threat.

Liquid nitrogen boils at 77 K. If it forms an ocean on a planet with a
nitrogen atmosphere (probably from evaporating the ocean), the average velocity
of the molecules in the atmosphere will be a sluggardly 237 meters per second. A
body with an escape velocity of a kilometer a second should be able to hold that
atmosphere, that is, an icy body roughly 2000 kilometers in diameter. Such an
object would be very far from its parent star, so even though liquid nitrogen
has a small liquid temperature range, temperatures on the object might not vary
all that much.

Names and Places

What is Sea Level?

Even on Earth, defining sea level is harder than you might think. Ocean
currents and weather distort the sea surface in ways having nothing to do with
the overall shape of the earth. When we say Mount Everest is 29,028 feet above
sea level, how do we know where the sea would be under Mount Everest? Not
only is the sea surface distorted into a bulging ellipsoid by the earth's
rotation, but masses within the earth create bumps and hollows. The shape the
sea would have if the earth were totally covered with water is called the
geoid. Its shape can be calculated from accurate measurements of the earth's
gravity.

For planets without water, elevation is measured from the surface of a sphere
with radius equal to the average radius of the planet. The planet will probably
also have an equatorial bulge, so the actual shape of the planet is approximated
with a flattened sphere called an ellipsoid. On the earth, the average elevation
is actually 2.5 kilometers below sea level.

Mapping lumpy and irregular bodies is an evolving field.

Places, Everyone

Every known body in the solar system rotates, so latitude and longitude are
defined just the way they are on the Earth. The poles are the points the planet
rotates around, and the equator is the circle midway between the poles.

For independently rotating planets, zero longitude, like zero longitude on
the earth, is arbitrary. Usually it is defined as the longitude that was facing
the earth or the sun at some specified time. But all satellites are locked to
their home planets, and zero longitude on these bodies is not arbitrary.
Zero longitude is the longitude facing the planet. East longitude is considered
positive, west longitude is negative.

A commission of the International Astronomical Union oversees naming, and the
names (so far) are recognized by the various spacefaring nations. The
rules are:

Features cannot be named after living people or people dead less than
three years.

Features cannot be named for political figures after 1800.

Features cannot be named for any religious figures from Christianity, Judaism, Islam,
Hinduism, Buddhism or Confucianism.

Sooner or later colonists on the Moon or Mars will want a Mount Putin or
Mount Reagan, and we will probably let them name things as they see fit. We can
deal with that issue when it happens.

Planetary geographical features have Latin names. Latin is traditional, apolitical, and
the closest thing to a universal language in history. Here are a few of the most
important, with the
literal Latin translation in parentheses, followed by the geographical meaning.

Linea (Line) Line or band

Macula (Spot) Dark spot (im-macula-te means "spotless")

Mons (Mountain) Mountain, plural Montes

Planitia (Plain) Low plain, basin

Planum (Plain) Plateau

Regio (Region) Region

Rupes (Cliff) Cliff or scarp

Vallis (Valley) Valley

What Is A Planet?

In ancient times there were seven planets, counting the Sun and Moon and
not counting the earth. After Copernicus and Kepler laid out the true nature
of planets, the number of planets dropped to six; the Earth was now in and the
Sun and Moon out. Uranus was discovered in 1781, raising the count to seven. The
huge gap between Mars and Jupiter inspired speculation that there were unknown
planets hidden there, and the first one, Ceres, was found in 1801. Three others
followed in the next six years, and the number of planets climbed to 11. But by
the mid-19th century minor planets were being discovered at an accelerating
rate. All were small, and it was obvious they weren't in the same league even as
Mercury, so a new category was created: minor planets. The known asteroids were
demoted and the planet count dropped back to 7. The discovery of Neptune in 1846
raised it to 8.

Neptune had been discovered because of its gravitational effect on Uranus.
Could there be more planets waiting to be discovered this way? Percival Lowell,
advocate of the "canals" on Mars, thought so, and launched a long campaign to
find one. Finally, in 1930, his assistant, Clyde Tombaugh, found a distant
object. It was called Pluto (the name was suggested by a ten-year old girl,
Venetia Burney) and raised the planet tally to nine.

Right from the git-go, Pluto was a troublemaker. It was tiny, much too small
to affect Neptune's orbit. But it could perhaps be as large as Earth, or at
least Mercury. But a near miss with a star as seen from Earth in the early
1970's showed Pluto must be smaller than the earth's moon. The discovery of
Pluto's satellite Charon in 1978 paradoxically strengthened and weakened Pluto's
status. Having a satellite strengthened the case for calling Pluto a planet, but
having a satellite also made it possible to calculate the combined mass of the
two objects, and together they turned out to be much smaller than even Mercury.

As astronomers discovered there were a host of objects in the outer solar
system, some rivaling Pluto in size, and that even minor planets have
satellites, the case for demoting Pluto gained momentum. In 2006 the
International Astronomical Union created a new category of planet, "dwarf
planet," defined as a body large enough for gravity to pull it into a sphere.
This condition is called "hydrostatic equilibrium," that is, the shape of the
planet is due mostly to gravity and rotation. Pluto, a couple of other distant objects, and the asteroid Ceres were designated
"dwarf planets."

The current definition of a true planet is, first, that it has to be massive
enough for gravity to pull it into a sphere (it can, of course, have an
equatorial bulge) and second, it has to be massive enough to "clear" a zone
around it. An asteroid orbiting a million miles inward or outward from the
earth's orbit would very soon have its orbit perturbed by close passages with
the Earth. But Pluto and Ceres aren't massive enough to prevent other objects
from having nearby orbits.

The diagram at left is a snapshot of the Solar System at a
particular time. The green dots are asteroids. Note that the asteroid
belt has a fairly sharp outer edge halfway between Mars and Jupiter and
a sharp inner edge just outside the orbit of Mars. Anything that tries
to orbit closer to either planet will soon have its orbit disturbed by
the gravitational pull of Mars or Jupiter. This is what it means for a
planet to "clear" its vicinity.

The purple dots are comets and the red
dots are asteroids that enter the inner Solar System. Neither group will
stay in those orbits very long. They will either hit a planet or have a
close encounter that radically changes their orbits.

This is not over yet. There are numerous large asteroids big enough to be
somewhat round that will challenge the definition of roundness as a criterion
for dwarf planets. And as we learn about solar systems outside our own, we will
surely discover newly formed systems with large objects that are planets by any
reasonable criterion but have not yet
cleared the space around them. Even something as small as Ceres or Pluto would
disturb the orbits of objects very close by, so the definition of how wide and
clear a "cleared zone" must be will need to be - ahem - cleared up. But since the definition of planets has changed
several times already, it should be no surprise that the definition may change
in the future.